The present invention relates to a pressure reducing device able to produce a pressure drop in an aerosol, as well as to an apparatus for the detection of aerosols and particularly aerosols of solid .alpha.-ray emitting particles using such a device.
It more specifically relates to an apparatus for the detection of aerosols, equipped with a pressure reducing device making it possible to produce a pressure drop in the measuring chamber, while ensuring the re-entrainment of the particles, which might be deposited on the walls of the pressure reducing device.
In general, .alpha.-ray emitting particle aerosols are detected on the basis of their being sampled on a filter. The latter is placed in a sealed chamber and faces the sensitive surface of a detector. Between the two surfaces a distance x is necessary for permitting the flow of air containing the aerosols without polluting the measuring head and for obtaining a homogeneous sample on the filter. However, when the distance x increases, the accuracy of the measurement decreases and the resolution of the detector becomes poor. It is then difficult to differentiate the measurement of the contaminants of a given energy (e.g. plutonium 238 and 239 isotopes) from that of other .alpha.-emitters constituting a background noise (e.g. solid daughter nuclei of radon, such as RaA and RaC'). Therefore an optimum distance is defined, which ensures that the flow of the aerosol and the performances of the measurement are satisfactory.
However, for a given source-detector distance, the resolution of the apparatus can be improved by introducing the aerosol into the measuring chamber at a pressure below atmospheric pressure.
Equipment exists, where the pipe for introducing the aerosol into the measuring chamber is equipped with a device able to introduce a pressure drop into the circuit, e.g. a valve with a regulatable throttle or constriction, thereby reducing the pressure in the measuring chamber.
However, even such a device has disadvantages, because a by no means negligible fraction of the aerosol particles can be retained in the pressure reducing device by adhering to its walls. The retained fraction, which varies with the size of the particles and with the flow rate, can reach 50% in the case of particles with a diameter greater than 1 .mu.m. Thus, the grain size distribution of the particles deposited on the filter is no longer representative of the aerosol, which falsifies the measurements.